[PATCH 1/3] gpu/drm: ingenic: Add trick to support 16bpp on 24-bit panels

[Paul Cercueil <paul@crapouillou.net>]

If the panel interface is 24-bit but our primary plane is 16bpp,
configure as if the panel was 18-bit. This tricks permits the display
of 16bpp data on a 24-bit panel by wiring each color component to the
MSBs of the 24-bit interface.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
---
 drivers/gpu/drm/ingenic/ingenic-drm.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ingenic/ingenic-drm.c b/drivers/gpu/drm/ingenic/ingenic-drm.c
index 6d47ef7b148c..034961a40e98 100644
--- a/drivers/gpu/drm/ingenic/ingenic-drm.c
+++ b/drivers/gpu/drm/ingenic/ingenic-drm.c
@@ -400,6 +400,8 @@ static void ingenic_drm_encoder_atomic_mode_set(struct drm_encoder *encoder,
 	struct drm_display_mode *mode = &crtc_state->adjusted_mode;
 	struct drm_connector *conn = conn_state->connector;
 	struct drm_display_info *info = &conn->display_info;
+	struct drm_plane_state *plane_state = crtc_state->crtc->primary->state;
+	const struct drm_format_info *finfo = NULL;
 	unsigned int cfg;
 
 	priv->panel_is_sharp = info->bus_flags & DRM_BUS_FLAG_SHARP_SIGNALS;
@@ -435,7 +437,22 @@ static void ingenic_drm_encoder_atomic_mode_set(struct drm_encoder *encoder,
 				cfg |= JZ_LCD_CFG_MODE_GENERIC_18BIT;
 				break;
 			case MEDIA_BUS_FMT_RGB888_1X24:
-				cfg |= JZ_LCD_CFG_MODE_GENERIC_24BIT;
+				if (plane_state && plane_state->fb)
+					finfo = plane_state->fb->format;
+
+				/*
+				 * If the panel interface is 24-bit but our
+				 * primary plane is 16bpp, configure as if the
+				 * panel was 18-bit. This tricks permits the
+				 * display of 16bpp data on a 24-bit panel by
+				 * wiring each color component to the MSBs of
+				 * the 24-bit interface.
+				 */
+				if (finfo &&
+				    finfo->format != DRM_FORMAT_XRGB8888)
+					cfg |= JZ_LCD_CFG_MODE_GENERIC_18BIT;
+				else
+					cfg |= JZ_LCD_CFG_MODE_GENERIC_24BIT;
 				break;
 			case MEDIA_BUS_FMT_RGB888_3X8:
 				cfg |= JZ_LCD_CFG_MODE_8BIT_SERIAL;
-- 
2.25.0